Barn find ST70

wildiowa wrote:Thank God there are people like you who have the ability, knowledge and equipment to do these kinds of things. This is why we have seen the development of such great amps and speakers and other stuff that is just wonderful equipment, both vintage and new. But...I must say I have absolutely no idea what you are up to here....you guys got so far into the weeds you lost me a long time ago but I am thoroughly entertained by the videos of your rebuild and testing being done here. I am just so far behind your curve it makes me realize how much I really don't know. But I still enjoy poking around under the hood.

Ive been working with others on a different forum where we went really deep into theory and math, Components on the PCB and measured to exact specification to see how each reacted with each other. Most of which would be well beyond the general restoration of this thread. This stuff got really deep including:The relationship between the values of capacitors and resistors to trace the low and high pass filtering properties of these components, where the filter response climbs in the frequency domain, its rate of change in DB over frequency and where it tapers off. An important component of frequency response.

Discussion of the Negative Feedback loops, How they affect the result, and why it is necessary to compromise higher frequency distortion in exchange for amplifier stability

Examination of stability as reflected by changing elements and re-evaluating a 5khz 1 watt square wave through the AMP

Limitations of the power transformer and how voltages drop (ever so slightly) as the transformer heats up towards stability. Also a relationship between power transformer fluctuation and the DC BIAS

Limitations of the GZ34 rectifier at full load, and the sag it produces.

If however, you do not want to get involved with a 2 month exercise of mathematics, Ill give you the cliff notes.

1. Replace the 2 green 50 uf caps with higher rated 100uf caps. Why, because back then, you couldnt get a 100uf cap that wasnt smaller than the size of a pipe bomb. Today you can, so take advantage of it2. There is nothing wrong with the selenium rectifier, if it works, leave it alone3. There IS something wrong with the capacitor can, replace it. If you arent willing to replace this DC leaking monster, dont bother with the rest of any work4. Every component should be checked, individually, most consumer meters can do this, so do it. EEEEEEVVVVVVRRRRRRYYYYY component5. If you are measuring the AMP at less than .3%THD. Make sure your microwave, AC, dishwasher, and other heavy load utilities are off (believe me)6. 1% tolerance means 1%, not 2%, however in most cases this means that the match on the PCP means 1% between the two components. the 1%47K would be fine with two 1%45Ks. But stick with the original plan7. The EL34's are balanced for current by channel not by tube. Matched tubes would be highly beneficial when measuring the 1.56v at the test port its good to know that each tube is pulling 50ma, not 60/408. To the distain of many in the lunatic fringe, the ST70 design is a sound design, even today, and need not be modified, at all, never, except for what I mentioned (technically not a mod but a repair)9. The idea of separate test ports for each tube using 10ohm resistors is also a good idea to see if the tube current is balanced in each channel10. When people describe an AMP characteristics and it sound like they are talking about wine, the artifacts they are describing is called distortion, they just dont know that11. These unique artifacts and harmonics (AKA distortion) is what makes a tube AMP a tube AMP. One were to prescribe to snake oil to somehow make their AMP 'better' just buy a solid state one, they're cheaper12. Fix it, enjoy it and be happy.13. Im sure you will hear how im completely wrong, and there is so much that can be done to improve the definition, and tighten the bass, and produce super fluffy sonic spacial kolidascopal artifacts. See 11

1. It's always a good idea to replace a selenium rectifier. As they age, the voltage drop increases. IN4007s are cheap and will last forever.

2. The capacitor can should be checked with a real capacitor tester. This is usually a tube based tester that can actually deliver 450 volts and measure the leakage at that tension. If it passes, keep it.

1. It's always a good idea to replace a selenium rectifier. As they age, the voltage drop increases. IN4007s are cheap and will last forever.

2. The capacitor can should be checked with a real capacitor tester. This is usually a tube based tester that can actually deliver 450 volts and measure the leakage at that tension. If it passes, keep it.

Otherwise, you are spot on.

1. Naturally, if the selenium rectifier was not producing the expected output DC with the correct input AC, then we would call it bad and replace it. I agree. However it definitely not stressed in this biascircuit and Ive rarely seen them fail. Also the voltage drop increase also increases resistance, so it gets hotter as it fails. I check them with a laser thermometer

2. I have a leakdown tester up to 600VDC. Most folks don't have one, of course. Ive come across many of these electrolytics, 99% of them have unacceptable leak, shorts or opens. That being said, the non-electrolytic capsin this unit are pretty resilient